In case of supplying a reformer with a hydrocarbon base fuel, for example a mixture gas of methane and water vapor [for example, steam/carbon (mole ratio)=about 2.5] and reforming the same into a hydrogen rich gas (endothermic reaction) through water vapor reforming, the operating temperature of the reformer is approximately 700° C. (exit temperature of reforming catalyst layer); but, the surface temperature of the material making the reformer is supposed to be equal or superior to about 1000° C. in the vicinity of the heating portion of the reformer.
There has been a problem that the surface of steel material making the reformer, exposed to an atmosphere of low oxygen concentration and/or high water vapor concentration under a high temperature, is oxidized by water vapor, a rough, porous and fragile iron oxide layer (supposed to contain Fe2O3 mainly) (referred to as red scale, hereinafter) is formed on the surface thereof, the red scale tends to progress on and on into the substrate, having a nature of peeling off easily from the substrate, and the more the red scale peels off from the substrate, the more the corrosion of steel material advances. Though the destruction of the material by corrosion may be mitigated by increasing the thickness of the steel material, the weight increases, the processing becomes difficult and the cost increases.
The conventional fuel reformer has been used a heat resistant alloy USUS310S or the like) or super alloy (Incoloy800 or the like) of Cr:20 mass %, Ni:18 mass % or more. However, even in a reformer made of super alloy, for instance, red scale appeared on the surface of the combustion gas passage side including the heating portion.
A fuel reformer material containing C 0.1 wt % or less, Ni 8 to 29 wt %, Nb 0.1 to 0.4 wt %, containing one or two elements chosen from Ti, Zr by 0.05 to 0.3 wt %, and further containing N 0.02 to 0.05 wt %, B 0.003 to 0.01 wt % and composed of remaining Fe and inevitable impurities (Japan Patent Publication Laid-Open HEI 5-339679); however, a problem of the generation of red scale was not considered.
The object of the present invention is to provide a light, low cost and cheap, highly reliable and long life fuel reformer, that resolves conventional problems, and produces no red scale through water vapor oxidation of the surface of steel material making the reformer, even when exposed to an atmosphere of low oxygen concentration and/or high water vapor concentration under a high temperature.